This invention relates to photographic cameras, and more particularly to electromagnetic control devices in such cameras. Still more particularly, it relates to the stabilization of an electric impulse for energizing a magnetic winding of such device.
Recently, the electromagnetic control devices have found wide use in photographic cameras. In still cameras having electrically timed shutters, this device is generally constructed to be responsive to the output of a timing circuit and arranged to control the closing movement of the shutter. In cine cameras, the period of actuation of the shutter drive means is controlled by an electromagnetic actuating device. In any case, efforts have been made to eliminate inconvenience in handling and portability of cameras. To do this it is necessary that the electrical power source or battery which is utilized in energizing a magnetic winding of the device be of small size so that the resulting output and capacity is small. This arises from the restriction of the space which the battery is intended to occupy. For this reason, it is of great importance to minimize consumption of electrical energy of the battery by the magnetic actuating device from the standpoint not only of preventing premature consumption of the battery but also of assuring a satisfactory dynamic range of exposure control even at lowered temperatures. The internal resistance of the battery is increased with decrease in temperature.
Energization of the magnetic winding continues during the period of actuation of the shutter, that is, from the initiation to the termination of an exposure. Hence the current flowing through the magnetic winding, however small its intensity may be, reaches a considerable level particularly when the shutter time is long. To avoid this, mechanical subsidiary means have been arranged to be moved away from the armature of the magnetic actuating device at an appropriate time interval before the termination of duration of an exposure time, while simultaneously permitting the start of energization of the magnetic winding. At that termination, the magnetic winding is de-energized in response to the output of a timing circuit, causing movement of the armature through which the shutter is closed. Such shutter arrangement gives rise to the problem of increasing the production cost of the camera owing to the resulting complicated mechanical construction. At the same time, it involves lowering the reliability of exposure control.
Attempts have been made to overcome the above-mentioned drawbacks by employing a permanent magnet in association with the electromagnet, as for example, disclosed in U.S. Pat. No. 4,020,433 (issued Apr. 26, 1977). Here the permanent magnet is arranged in the magnetic circuit of the device so that the armature is attracted by the magnetic flux of the permanent magnet. When the magnetic winding is energized, this magnetic flux is cancelled to permit movement of the armature under the action of a bias spring by which the shutter is driven to operate. Thus, remarkable reduction of consumption of the electrical energy can be achieved without an unduly large increase in complexity of mechanical construction and arrangement of the shutter control. The use of the permanent magnet leads to the production of what is called a moving coil type magnetic actuating device in which the magnetic winding, upon energization, is moved relative to the permanent magnet. Such movement of the magnetic winding is translated to control actuation of the shutter, or a trigger for the various portions of the camera mechanism.
With these two conventional types of magnetic actuating devices, though the efficiency of usage of the electrical energy is remarkably improved as compared with the aforesaid overallperiod power supply type magnetic actuating device, the magnetic resistance in the magnetic circuit is substantially larger than that in the overall-period power supply type because of the permanent magnet or the air gap which is comparatively large in the magnetic circuit. In consequence, the required value of magnetomotive force exerted in the magnetic winding is increased to an amount available from the battery which is adapted for use in the camera since the internal resistance of the battery is relatively large. A control circuit for the magnetic actuating device is, therefore, designed to include a capacitor on which a certain fraction of the electrical energy necessary to energize the magnetic winding is previously stored. Thus energization of the magnetic winding takes the form of an electric pulse or impulse.
The time lag from the moment at which an actuating signal has been produced by the timing circuit to the moment at which the armature or the moving coil starts to move varies, depending upon the magnitude of the electric impulse. The present inventors have now found from many experiments and analysis that, with the moving-coil type actuating device, when the voltage of the battery is varied by 2 volts, the variation of the time lag reaches 1 millisecond, as shown by a dashed line curve in FIG. 1. With the armature type actuating device, the variation of the time lag ranges from 0.2 to 0.5 milliseconds, as shown by a solid line curve.
Since the response of the composite magnet depends upon the various parameters such as magnetic permeability and saturation flux density characteristic of the magnetic circuit, and the response of the driven mechanism depends upon the force of the spring means, the inertia and the like, the range of variation of the time lag can be reduced to some extent by proper selection of these design parameters. Because of the presence of the comparatively large air gap of the permanent magnet in the magnetic circuit, and, therefore, of the creation of the extremely large magnetic resistance as compared with the overall-period power supply type magnetic actuating device, the degree of freedom of parameter-selection is extremely restricted. This makes it difficult to maintain the response of the entire system at a certain constant level as the actual voltage of the battery varies.
This problem becomes serious when a highly accurate exposure control is desired even at faster shutter speeds, or when such magnetic control device is utilized in adjusting the size of diaphragm aperture. Particularly with a camera having various electronic control devices supplied with electrical power from a common battery, various photographic conditions which may be encountered result in a large difference in the amount of electrical energy used up at a time. This leads to a large range of variation of the voltage across the terminals of the battery due to the internal resistance thereof. Further, this internal resistance is varied with ambient temperature, thus the accuracy of exposure control is substantially decreased.